Applied XML Programming for Microsoft .NET.pdf - Csbdu.in

The NameTable ObjectOne of the secrets behind the XML readers' great performance is the NameTableclass—a helper class that works as a quickly accessible table of string objects. Several.NET classes, including, but not limited to, XmlDocument and XmlTextReader, makeuse internally of a NameTable object. User applications too can use a NameTableobject to store potentially duplicated strings more efficiently. When stored in a nametable, a string is said to be an atomized string.The net effect of atomized strings is that XML readers can manage elements andattributes as references rather than values and can therefore function more effectively,especially in terms of memory occupation and speed of comparison. Comparing twoobject references is much faster than comparing all the characters that form a string.The NameTable class, which inherits from the abstract class XmlNameTable, has arelatively simple programming interface and provides methods to add new items and toread them back. You add a new item to a name table using the Add method.NameTable table = new NameTable();string name = table.Add("Author");You get the atomized string with the specified value from the table using the Getmethod.string name = table.Get("Author");XML reader classes make internal use of name tables. The reader's name table can beaccessed through the NameTable property. The reader's name table contains an atom(a reference to the string object) for each distinct element or attribute name, completedwith namespace information for uniqueness. If the XML document being processedcontains, say, 1000 nodes named , only one atomized entry will be createdin the name table. Don't mistake the NameTable object for a worker table in which thereader stores all the document's nodes. Instead, the NameTable object is just a workercollection of unique names stored in a way that allows for more effective storage,retrieval, and comparison.The NameTable object is internally implemented using an array of structures thatmimics a hash table. Like a hash table, the array manages strings using hash codes.So when a new string is added to the table, a new hash code is generated andcompared to the others existing in the array. If a string with that hash code alreadyexists in the table, a reference to the existing atom is returned; otherwise, a new entryis created and the relative reference (atom) returned. In case of overflow, the size of thearray is doubled.The NameTable object uses a homemade hash table rather than the official .NETHashTable object because the HashTable object is not as simple and compact asrequired in this context.When creating a new instance of the XmlTextReader class, you can also indicate thespecific NameTable object to use.Designing a SAX Parser with .NET ToolsAs mentioned in Chapter 1, significant differences exist between .NET XML readers—akind of cursor-like parser—and Simple API for XML (SAX) parsers. All of thesedifferences can be traced, directly or indirectly, to the differences existing between thepush model, which is typical of SAX, and the pull model on which readers are based.A SAX parser takes full control over the parsing process, extrapolates any predefinedpiece of XML code, duplicates it into local buffers, and finally pushes that data down tothe calling application. The interaction between the parser and the application takesplace through application-defined classes that, in turn, implement SAX-definedinterfaces.39